The union said up to 4,000 workers will be involved, at 373 Crown offices - the larger sites usually situated in high streets.

The action is in protest at plans to close or franchise more than 70 Crown offices, as well as over pay and jobs. A number of Crown offices have already been franchised to firms such as WH Smith.

The union said Post Office staff have not received a pay rise since April 2011.

CWU deputy general secretary Dave Ward said: "With Post Office management refusing to negotiate, and with such a huge vote in favour of strike action, we have had no choice but to serve notice for strike action.

"The patience and loyalty of our Post Office members has been abused by Post Office management. This dispute is about protecting jobs and services as well as securing a fair pay increase for our members. We're saying to the public 'If you value your post office, support these workers on Easter Saturday and beyond'."

The Post Office said it was "disappointed" with the strike, adding that the CWU refused to understand the requirement for "critical change" across the Crown network which is losing £40 million a year.

The company said strike action would do nothing to change the need to drive forward its transformation strategy aimed at building a "profitable, modern and commercially viable" Crown network that is not reliant on Government funding.

The Post Office said it had offered a series of cash payments of up to £3,400 to be paid before April 2015. Contingency plans are in place to ensure that any disruption to customers is kept to an "absolute minimum".